video: ipu: initialize g_ipu_clk, g_ldb_clk statically

When the FB driver has not been initialized, booting Linux will hang
the system when ipuv3_fb_shutdown() is called from arch_preboot_os()
because clk_enable(g_ipu_clk) is a NOP with g_ipu_clk == NULL.
There is no need to initialize these variables at runtime, so use
static initialization to prevent this problem.